Condensed Consolidated Balance Sheets - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 10.1	$ 20.9
Accounts receivable, less allowance for doubtful accounts	417.9	350.4
Production fuel, at weighted average cost	47.0	61.4
Gas stored underground, at weighted average cost	15.9	49.0
Materials and supplies, at weighted average cost	105.1	101.4
Regulatory assets	65.8	79.8
Other	153.4	122.2
Total current assets	815.2	785.1
Property, plant and equipment, net	12,665.2	12,462.4
Investments:
ATC Holdings	295.0	293.6
Other	138.6	137.7
Total investments	433.6	431.3
Other assets:
Regulatory assets	1,726.6	1,657.5
Deferred charges and other	72.8	89.7
Total other assets	1,799.4	1,747.2
Total assets	15,713.4	15,426.0
Current liabilities:
Current maturities of long-term debt	256.5	256.5
Commercial paper	514.7	441.2
Accounts payable	387.7	543.3
Regulatory liabilities	179.8	142.7
Other	249.0	260.4
Total current liabilities	1,587.7	1,644.1
Long-term debt, net (excluding current portion)	5,362.2	5,246.3
Other liabilities:
Deferred tax liabilities	1,642.0	1,603.1
Regulatory liabilities	1,302.6	1,350.5
Pension and other benefit obligations	493.6	509.1
Other	442.9	287.2
Total other liabilities	3,881.1	3,749.9
Commitments and contingencies (Note 14)
Common equity:
Common stock	2.4	2.4
Additional paid-in capital	2,100.0	2,045.5
Retained earnings	2,587.3	2,545.9
Accumulated other comprehensive income	2.4	1.7
Shares in deferred compensation trust - 375,542 and 384,580 shares at a weighted average cost of $25.93 and $25.60 per share	(9.7)	(9.8)
Total common equity	4,682.4	4,585.7
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	4,882.4	4,785.7
Total liabilities and equity	15,713.4	15,426.0
IPL [Member]
Current assets:
Cash and cash equivalents	3.9	9.7
Accounts receivable, less allowance for doubtful accounts	209.5	153.5
Production fuel, at weighted average cost	31.2	44.8
Gas stored underground, at weighted average cost	5.4	26.1
Materials and supplies, at weighted average cost	56.9	55.4
Regulatory assets	27.0	39.2
Other	21.3	43.1
Total current assets	355.2	371.8
Property, plant and equipment, net	6,947.9	6,781.5
Other assets:
Regulatory assets	1,324.4	1,239.8
Deferred charges and other	19.5	18.3
Total other assets	1,343.9	1,258.1
Total assets	8,647.0	8,411.4
Current liabilities:
Commercial paper	0.0	50.4
Accounts payable	227.3	304.9
Regulatory liabilities	107.3	90.0
Other	150.3	161.8
Total current liabilities	484.9	607.1
Long-term debt, net (excluding current portion)	2,667.8	2,552.3
Other liabilities:
Deferred tax liabilities	970.9	957.3
Regulatory liabilities	641.5	664.9
Pension and other benefit obligations	174.6	178.4
Other	365.3	220.7
Total other liabilities	2,152.3	2,021.3
Commitments and contingencies (Note 14)
Common equity:
Common stock	33.4	33.4
Additional paid-in capital	2,322.8	2,222.8
Retained earnings	785.8	774.5
Total common equity	3,142.0	3,030.7
Cumulative preferred stock of Interstate Power and Light Company	200.0	200.0
Total equity	3,342.0	3,230.7
Total liabilities and equity	8,647.0	8,411.4
WPL [Member]
Current assets:
Cash and cash equivalents	3.4	8.7
Accounts receivable, less allowance for doubtful accounts	198.7	190.1
Production fuel, at weighted average cost	15.8	16.6
Gas stored underground, at weighted average cost	10.5	22.9
Materials and supplies, at weighted average cost	45.3	42.9
Regulatory assets	38.8	40.6
Other	103.5	62.8
Total current assets	416.0	384.6
Property, plant and equipment, net	5,322.8	5,287.3
Other assets:
Regulatory assets	402.2	417.7
Deferred charges and other	24.0	62.9
Total other assets	426.2	480.6
Total assets	6,165.0	6,152.5
Current liabilities:
Current maturities of long-term debt	250.0	250.0
Commercial paper	138.4	105.5
Accounts payable	104.5	180.9
Regulatory liabilities	72.5	52.7
Other	112.5	105.5
Total current liabilities	677.9	694.6
Long-term debt, net (excluding current portion)	1,585.4	1,584.9
Other liabilities:
Deferred tax liabilities	601.0	582.0
Regulatory liabilities	661.1	685.6
Finance lease obligations - Sheboygan Falls Energy Facility	57.9	60.0
Pension and other benefit obligations	214.2	217.7
Other	188.3	178.2
Total other liabilities	1,722.5	1,723.5
Commitments and contingencies (Note 14)
Common equity:
Common stock	66.2	66.2
Additional paid-in capital	1,309.0	1,309.0
Retained earnings	804.0	774.3
Total common equity	2,179.2	2,149.5
Total equity	2,179.2	2,149.5
Total liabilities and equity	$ 6,165.0	$ 6,152.5